(a) Field of the Invention
The present invention relates to a method for forming a pattern on a chemical sensitization photoresist and, more particularly, to a method for forming a pattern on a chemical sensitization photoresist to obtain a photoresist mask used as an etching mask or an ion-implantation mask on a semiconductor substrate.
(b) Description of the Related Art
A higher integration and a higher-speed operation have been achieved in the field of semiconductor devices by obtaining finer patterns for device element using a photolithographic technique. The conventional semiconductor devices using a design rule of 0.5 .mu.m are fabricated by g-line lithography using an exposure wavelength of 436 nm, whereas semiconductor devices using a design rule of 0.5 to 0.35 .mu.m are fabricated by i-line lithography using an exposure wavelength of 365 nm.
Next generation semiconductor devices to be developed in the near future may be fabricated in a design rule of 0.30 to 0.18 .mu.m. A KrF excimer laser lithography using an exposure wavelength of 248 nm is expected to provide such a finer design rule.
Conventional photolithographic techniques generally provided patterns of respective design rules larger than the exposure wavelength used in the photolithographic techniques. On the other hand, the KrF excimer laser is expected to provide a pattern in a finer design rule of 0.18 .mu.m which is smaller than the wavelength 248 nm of the KrF excimer laser. In such a case, problems of insufficient resolution and depth of focus should be considered.
The resolution R and the depth of focus (DOF) are represented as follows: EQU R=k.sub.1.multidot..lambda./NA,
and EQU DOF=k.sub.2.multidot..lambda./(NA).sup.2,
wherein .lambda. and NA are exposure wavelength and numerical aperture, respectively, of the exposure system, and k.sub.1 and k.sub.2 are constants.
In the above formulae, the resolution R can be improved by employing a smaller exposure wavelength and a larger numerical aperture, whereas the DOF suddenly decreases along with the smaller exposure wavelength and the larger numerical aperture. That is, the resolution R and DOF in the exposure system are tradeoffs.
In addition, it is generally difficult in a KrF excimer laser lithography to use a positive type photoresist, obtained as a compound of novolac resin and naphthoquinone-diacido, which has been used in the g-line and i-line lithographies. This is because novolac resin has a large optical absorption for light having wavelength of 248 nm, and cannot provide a suitable profile having a sharp vertical edge.
In view of the above, active researches have been conducted to obtain a new type photoresist that is called chemical sensitization photoresist. The chemical sensitization photoresist is made from a base resin, an acid generator which generates acid during exposure thereof, and a compound having a protective group which changes solubility of the compound for a developing solution depending on the amount of acid. The base resin is generally selected from phenolic resins having a high transmission factor to light having a 248 nm wavelength.
As one of the techniques for forming a finer pattern using a chemical sensitization photoresist that exceeds the conventional patterning technique, Patent Publication JP-A-4-364021 proposes a method for heating the photoresist up to around the softening point thereof after the patterning thereof to swell the photoresist to obtain a finer pattern.
In the proposed technique, however, the heating of the photoresist up to the softening point thereof causes problems of degradation in the profile and insufficient controllability of the dimensions in the resultant photoresist pattern.